1. Field of the Invention
The invention generally relates to charge pumps, and more particularly, to methods and apparatus for generating a high positive or a negative voltage for memory devices.
2. Description of the Prior Art
The era of digital information has arrived which pushed the development of electronic information processing devices such as computers, wireless devices, personal digital assistants (PDAs), portable multimedia players/recorders, and the like in the recent years. One crucial component to any electronic information processing device is the memory device which has gone under substantial advancement. The performance in speed and reliability along with the size and packaging of these memory devices have greatly improved as a result smaller and faster memory devices are continuously being introduced to the market.
In order to reduce power consumption and extend battery life, much of the integrated circuitry such as memory devices used in portable devices is being designed to run at low voltage levels. This reduces the power usage and reduces the heat generated by the circuit components allowing more components to be placed closer to one another. The circuitry and components used in portable computers typically are being designed to operate at voltages levels substantially less than the previous standard of 5V, with 1.0V and lower becoming increasingly common.
A major problem is that conventional charge pumps have difficulty dealing with the lower battery voltages being used. In particular, the MOS transistors used in the charge pumps have switching threshold voltages that are a large fraction of the supply voltage. The problem is related to the fact that diode-connected transistors develop increasing back-bias between the source and the body of the transistor as the voltage increases along the length of the pump. The result of this back-bias (also known as the “source-body effect”, “M factor”, or “body effect”) is to increase the effective threshold of the transistor, in some higher voltage cases almost doubling it. With increased effective transistor thresholds and decreased supply voltages, the charge pump transistors would no longer switch properly and the charge pump would not work.
Many designs used a technique called “bootstrapping” to generate higher amplitude clock signals to compensate for the increased effective threshold voltages relative to the supply voltage. The bootstrapping technique involves the use of a charge capacitor that charges on every clock pulse and discharges between pulses, adding the discharged voltage to the original input voltage of the bootstrapping circuit so the output could be multiplied to a number of times the original input. Applying a uniform high clock voltage, generated by bootstrapping, leads to energy inefficiency because the greater the current delivered by the clocking voltage, the less efficient the bootstrapping operation. In the latter stages where high voltages are required, this inefficiency was unavoidable. In the initial stages of the charge pump, where as high a voltage is not needed, the clock bootstrapping operation was inefficient.
In general, currently available charge pumps are inefficient, large, and complex. They do not properly deal with low initial supply voltages and fail to address the problems inherent with higher threshold voltages caused by the body effect. A solution, which would provide a simple charge pump with efficient operation using a low initial supply voltage, has long been sought but has eluded those skilled in the art. As the popularity grows of portable battery-powered devices in which such a design could be particularly useful, it is becoming more pressing that a solution be found.
Different approaches to designing charge pumps were previously disclosed. “Charge pump circuit having a boosted output signal” U.S. Pat. No. 4,935,644 by J. Tsujimoto shown in FIGS. 1 and 2 discloses a charge pump circuit having a first charge pump circuit and a second charge pump circuit that is controlled by two phase clocks. The first and the second charge pump circuits comprise a series of serially diode-connected MOSFETs where the gates of the transistors in the second charge pump are connected to the corresponding stage transistors in the first charge pump. The final output of the first charge pump circuit is connected to a high resistance element so there is no external loss and the second charge pump circuit can achieve an adequate boost-up voltage where there is a greater external current loss. Tsujimotos charge pump circuit uses two identical but out-of-phase clocks and no substrate connection. “Method and apparatus for a two phase bootstrap charge pump” U.S. Pat. No. 5,462,469 by Tedrow shown in FIGS. 3 and 4 discloses a two phase bootstrap charge pump that uses four complicated and non-identical clock signals to pre-charge the gates and transfer charge to the next stage. Tedrows two phase bootstrap charge pump does not have any substrate connection to remove body effects and uses NMOS for positive pump.